Sedimentary gold-bearing ores containing indigenous organic carbonaceous material and gold bearing sulfidic minerals are notoriously refractory to standard cyanidation treatment for the recovery of their gold content. Investigation into the cause of this problem has indicated that the carbonaceous materials comprises active carbon and long-chain organic compounds. The active carbon appears to adsorb the gold cyanide complex [Au(CN).sub.2 --] from cyanide leaching solutions and the long-chain organic compounds appear to form stable complexes with the gold. In addition, some of these ores also contain gold-bearing sulfides. The sulfides contain gold either as a mechanical inclusion, or as atoms of gold included in the sulfide crystal lattice.
In order to overcome this sequestering of the gold and to render the gold component of the ore more amenable to standard cyanidation treatment it has heretofore been proposed that the ore be subjected to a preliminary oxidation treatment to oxidize the carbonaceous, sulfidic mineral materials, and as much of the carbonaceous mineral materials as can be oxidized. In experiments conducted by the United States Bureau of Mines, carbonaceous gold-bearing ores were subjected to a preliminary oxidation with a wide variety of reagents including ozone, sodium hypochlorite, calcium hypochlorite, permanganates, perchlorates, chlorates and oxygen prior to subjecting the oxidized ore to standard cyanidation treatment to extract the gold content therefrom. Of these preliminary oxidation treatments, it was found that the hypochlorites appeared to be the most effective.
In U.S. Pat. No. 3,846,124 to Wilbur J. Guay it was shown that the recoverability, by standard cyanidation, of the gold content of carbon-containing sedimentary gold-bearing ores is increased by subjecting the ore to a preliminary oxidation treatment in which chlorine gas is introduced into an aqueous slurry of the ground ore to the extent that the slurry will adsorb the chlorine, the thus treated slurry being maintained at the chlorination treatment temperature of about 70.degree. to 85.degree. F. for at least 6 hours. The oxidized ore is then subjected to standard cyanidation to extract the gold content therefrom with gold recoveries in the order of 75% or more of the gold content of the ore.
In U.S. Pat. No. 4,038,362 to Wilbur J. Guay it was shown that the recoverability, by standard cyanidation, of the gold content of sedimentary gold-bearing ores containing organic carbonaceous material and gold-bearing pyrite, or other gold-bearing sulfides, is increased and the cost of the pretreatment greatly reduced by subjecting the ore to a two-stage preliminary oxidation treatment. In this process an aqueous slurry of the ore is first heated to about 167.degree. to 212.degree. F., and air or oxygen is then introduced into the heated slurry to oxidize and eliminate a substantial portion of the carbonaceous material and oxidizable sulfides in the slurried ore. The slurry is then cooled to about 70.degree. to 85.degree. F., and chlorine gas is introduced into the slurry to substantially complete the oxidation and chlorination of the carbonaceous content of the slurried ore. The oxidized ore is then subjected to conventional cyanidation to recover the gold content thereof.
Conventional cyanidation practice involves leaching the finely divided gold-bearing ore at ambient temperatures with an aqueous solution containing from about 0.01 to 0.1 percent by weight of an alkali metal cyanide and, usually, a lesser amount of an alkali metal hydroxide to form a water soluble aurocyanide complex. However, the sedimentary gold-bearing ores with which the present invention is concerned contain a small but significant amount of elemental carbon that is not oxidized by the preliminary oxidation treatment of the ore and that tends to adsorb a portion, and with some ores a major portion, of the aurocyanide complex that is produced in the course of the cyanidation treatment, the adsorbed gold complex being discarded with the ore tailings following the cyanidation treatment.
It is known that the aurocyanide complex can be desorbed from manufactured activated carbon with cyanide solutions containing much higher strengths of cyanide (in the order of from 0.1 to 1.0 percent by weight alkali metal cyanide and from 0.1 to 10 percent by weight alkali metal hydroxide) at substantially higher temperatures than are usually used in the standard gold cyanidation process. I have now made the surprising discovery that the gold content of ores containing adsorptive carbon can be leached and recovered therefrom at the high cyanide strengths used for desorbing gold from manufactured activated carbon at temperatures from about 167.degree. to 212.degree. F. Under these conditions, the gold content of the ore will dissolve and the resulting aurocyanide complex will be adsorbed by the adsorptive carbon component of the ore until chemical and physical equilibrium are reached between the gold content of the ore, the aurocyanide complex in solution and the aurocyanide complex adsorbed on the adsorptive carbon component of the ore. The alkali cyanide and the aurocyanide complex are protected from decomposition at these temperatures by maintaining much more strongly alkaline conditions than are used in standard cyanidation of gold ores. After reaching or nearly reaching chemical equilibrium, the solution can be separated from the ore and the reaction repeated with fresh cyanide solution containing no gold until the gold which is dissolved from the ore in the second cyanide treatment again approaches equilibrium with the adsorbed gold. The procedure can be repeated as many times as necessary in order to get the desired recovery of gold from the ore. Alternatively, particles of activated carbon can be introduced into each stage of gold dissolution. At the end of each stage, equilibrium is reached or nearly reached between the gold in solution, the gold adsorbed on the added particles of activated carbon, and the gold adsorbed by the adsorptive carbon component of the ore. The added particles of activated carbon can be separated from the aqueous ore slurry by passing the slurry over a vibrating sieve which retains the carbon particles while allowing the ore slurry to pass therethrough. The carbon particles are then treated to recover the aurocyanide complex which has been adsorbed on the surfaces thereof.
Gold ores containing very adsorptive carbonaceous materials, ores from which little or no gold can be recovered by ordinary methods of cyanidation due to the adsorption of the aurocyanide complexes on the carbon component of the ore, have shown recoveries of from 80 to 90% of the gold content of the ore after only four stages of treatment with the strong cyanide solutions employed in the practice of the present invention. These recoveries have been achieved by both of the procedures just described--namely, separation of the gold bearing solutions from the ore after each dissolution stage, or adsorption of the aurocyanide complex on granular activated carbon added to the slurry of ore and cyanide solution during each dissolution stage followed by removal of the added carbon granules and recovery of the gold adsorbed thereon. Moreover, using these new procedures instead of chlorination of the ore followed by standard cyanidation results in significantly improved process economics, particularly with ores that consume large amounts of chlorine per ton of ore treated.